I. Field of the Invention
The present invention relates generally to multiple-access spread-spectrum communication systems and networks. More specifically, the present invention relates to a method and apparatus for acquiring signals in such systems.
II. Description of the Related Art
In general, wireless communications systems can be terrestrial or satellite-based. An examplary terrestrial wireless communications system includes at least one terrestrial base station and at least one user terminal (for example, a mobile telephone). The base station provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system. An examplary satellite-based wireless communications system includes at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system. Some of these wireless communication systems employ spread spectrum techniques.
In a typical spread-spectrum communications system, a set of preselected pseudorandom noise (PN) code sequences is used to modulate (i.e., xe2x80x9cspreadxe2x80x9d) information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communications signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In a satellite forward communications link (that is, in a communications link originating at a gateway and terminating at a user terminal), PN spreading codes are used to discriminate between signals transmitted by a gateway over different beams, and to discriminate between multipath signals. These PN codes are usually shared by all communications signals within a beam.
In an examplary CDMA spread-spectrum system, xe2x80x9cchannelizingxe2x80x9d codes are used to discriminate between signals intended for particular user terminals (hereinafter referred to as xe2x80x9ctraffic signalsxe2x80x9d) transmitted within a satellite beam or sub-beam on the forward link. The channelizing codes form orthogonal channels in a subbeam over which communication signals are transferred. That is, a unique orthogonal channel is provided for each user terminal on the forward link by using a unique xe2x80x9cchannelizingxe2x80x9d or covering orthogonal code to modulate signals intended for that user terminal. Walsh functions are generally used to implement the channelizing codes, also known as Walsh codes or Walsh sequences, with a typical length being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems.
Typical CDMA spread-spectrum communications systems contemplate the use of coherent modulation and demodulation for forward link user terminal communications. In communications systems using this approach, a xe2x80x9cpilotxe2x80x9d carrier signal (hereinafter referred to as a xe2x80x9cpilot signalxe2x80x9d) is used as a coherent phase reference for forward links. That is, a pilot signal, which contains no data modulation, is transmitted by a gateway throughout a region of coverage. A single pilot signal is usually transmitted by each gateway for each beam used for each frequency used. These pilot signals are shared by all user terminals receiving signals from the gateway on a given beam.
Pilot signals are used by user terminals to obtain initial system synchronization and for time, frequency, and phase tracking of other signals transmitted by the gateway. Phase information obtained from acquiring and tracking a pilot signal is used as a carrier phase reference for coherent demodulation of other system signals or traffic signals. This technique allows many signals to share a common pilot signal as a phase reference, providing for a less costly and more efficient tracking mechanism. In addition to pilot signals, there are other shared resources such as paging and synchronization signals used to transmit system overhead information and specific messages to user terminals.
Because a pilot signal usually does not involve data modulation, a spread-spectrum pilot signal can be characterized as a carrier signal modulated by a PN spreading code. In one approach, all pilot signals within a communications system use the same PN spreading code or set of codes, but each beam uses a different relative code timing offset. This provides signals that can be readily distinguished from each other while providing simplified acquisition and tracking. In another approach, each pilot signal can be generated using a different PN code.
CDMA systems require rapid and accurate acquisition of the timing of the pilot PN sequence. The timing of the PN sequence must be known accurately in order to obtain information from modulated signals. The acquisition of this timing must be rapid to avoid delaying the acquisition of communication signals or establishing communication links which could unacceptably frustrate system-users.
The search process involves generating timing hypotheses and testing each hypothesis to ascertain which provides the most likely or best match to the signal timing, sometimes referred to as finding the xe2x80x9ccorrectxe2x80x9d hypothesis. One common design objective, however, is to reduce the complexity and cost of the user terminal. Therefore, a minimum amount of resources may be provided, including circuitry or processing capability. For this reason, some conventional searchers test the hypotheses serially, eliminating each less likely or incorrect hypothesis before testing the next. However, this approach is also very time-consuming, potentially delaying signal acquisition an unacceptable amount.
Another conventional approach is to test the hypotheses in two stages. In the first stage, the least likely hypotheses are eliminated. In the second stage, each remaining hypothesis is tested serially. While this approach requires less time than the single-stage approach described above, it is still very time-consuming.
What is needed is a technique and apparatus to rapidly test hypothesis and quickly acquire pilot, and, therefore, other communication signals.
In one aspect the present invention provides a user terminal parallel searcher comprising a pseudonoise (PN) code generator for generating at least one pseudonoise sequence, a plurality of slices or slice processing elements, and a non-coherent accumulator that accumulates the output of each slice non-coherently to produce a non-coherent sum for each slice. Each slice includes a delay unit for delaying the pseudonoise sequence by a predetermined chip time, a despreader for operating on a data stream as a function of a previously delayed pseudonoise sequence, and at least one coherent accumulator for coherently accumulating an output of the despreader. An output from the pseudonoise generator is coupled to an input of the of the first slice. Each slice receives an output from the delay unit of the previous slice.
In another aspect, the present invention provides a method for detecting the presence of a signal in a data stream, where the signal has been spread using a pseudonoise sequence. The method comprises the steps of delaying a pseudonoise sequence by a plurality of predetermined delays to produce a plurality of delayed pseudonoise sequences, combining each of the delayed pseudonoise sequences with the data stream to produce a plurality of despread data streams, and accumulating each of the despread data streams for a duration to produce a plurality of coherent sums.
One advantage of the present invention is that it permits a receiver to search many PN offsets simultaneously, thereby reducing overall signal acquisition time. The invention also allows a receiver to search over multiple PN codes, multiple orthogonal codes or pilot channels, and multiple frequency ranges, each of which provides receiver with increased flexibility and usefulness, especially for advanced communication system designs. This process is not adversely affected by the presence of an outer code in multi-layered code environments.
Another advantage of the present invention is that it reduces the amount of resources required to perform non-coherent accumulation and other operations in a searcher receiver.